In the operation of electricity utility (or public) grids, hereinafter also called electrical energy supply systems or simply “systems”, it is occasionally desirable to be able to switch off the entire system or parts thereof for all consumers or individual consumers or to maintain a switched-off or disconnected condition after a failure or breakdown (loss of mains) in order to assure a voltage-free state for carrying out maintenance or repair work. This is not easily possible if the grid or system is supplied not only by a central energy supply plant, but also by decentralized voltage sources such as photovoltaic plants, since even in the event that an operator of the central plant executes a system shutdown or the like, these decentralized voltage sources continue to supply energy into the system and therefore lead to a local island formation (islanding).
When there are decentralized voltage sources, which, in the event of a system disconnection or the like, are not sufficient to cover the existing energy demand and therefore in particular suffer a drop in voltage, an undesirable island formation can be avoided by monitoring the voltage and automatically shutting down the relevant decentralized plant if the voltage falls below a predetermined minimum voltage. However, if the decentralized voltage source is in a position to continue maintaining the system voltage within strict tolerances with regard to amplitude and frequency even in the event of a partial or total outage or shutdown of essential components, then the above-described monitoring method alone would not be able to cause the decentralized voltage source to shut down.
In order to prevent local islanding from being produced and posing a threat to people and/or electrical systems even in such a case, methods and apparatuses of the type described at the beginning have become known. They serve the purpose of detecting unusual impedance changes in the public grid and shutting down associated decentralized voltage sources when critical values are reached. As a rule, a shutdown is required only if an impedance increase of 0.5 Ω or more (VDE 0126) occurs in the system. It is assumed that such impedance changes do not occur during normal system operation and are therefore characteristic of the outage or the desired shutdown of a transformer station or the like. A further requirement is that the shutdown occurs within a period of at most five seconds after the shutdown or outage of the system. This makes a continuous or quasi-continuous monitoring and measurement of the system impedance necessary.
With the use of known methods and apparatuses of the types mentioned at the beginning, either high and/or pulse-shaped testing currents with system frequency are fed into the system or testing impedances in the form of capacitors or transformers are connected into the system in rapid succession in order to obtain information from the thus modified system currents and system voltages about the system impedance, which is usually dependent on the frequency (DE 36 00 770 A1, DE 195 04 271 C1, DE 195 22 496 C1, DE 100 06 443 A1). However, such methods and apparatuses are accompanied by a variety of undesirable problems. For example, if it is necessary to use a comparatively high, possibly pulse-shaped testing current in order to analyze the system voltage with regard to its base or fundamental frequency or its harmonic oscillations, then a powerful interference is disadvantageously produced in the system voltage, which can lead to an erroneous interpretation of measurement results. On the other hand, the use of a test impedance, for example, requires a costly set of power electronics. Finally, if broadband testing currents are provided, then it is necessary to analyze the system voltage over a larger frequency spectrum, thus requiring the provision of substantial computing power and therefore high-quality microprocessors or digital signal processors. Apart from this, a broadband testing current causes so many undesirable interferences in the system that it is hardly possible to distinguish the expected test response from among these interferences.